The activation of Rac1 by M3 muscarinic acetylcholine receptors involves the translocation of Rac1 and IQGAP1 to cell junctions and changes in the composition of protein complexes containing Rac1, IQGAP1, and actin

The Evaluation of Rac1 Signaling as a Potential Therapeutic Target of Alzheimer's Disease

The Small GTPase Rac1 is critical for various fundamental cellular processes, including cognitive functions. The cyclical activation and inactivation of Rac1, mediated by Rac guanine nucleotide exchange factors (RacGEFs) and Rac GTPase-activating proteins (RacGAPs), respectively, are essential for activating intracellular signaling pathways and controlling cellular processes. We have recently shown that the Alzheimer's disease (AD) therapeutic drug donepezil activates the Rac1-PAK pathway in the nucleus accumbens (NAc) for enhanced aversive learning. Also, PAK activation itself in the NAc enhances aversive learning. As aversive learning allows short-term preliminary AD drug screening, here we tested whether sustained Rac1 activation by RacGAP inhibition can be used as an AD therapeutic strategy for improving AD-learning deficits based on aversive learning. We found that the RacGAP domain of breakpoint cluster region protein (Bcr) (Bcr-GAP) efficiently inhibited Rac1 activity in a membrane ruffling assay. We also found that, in striatal/accumbal primary neurons, Bcr knockdown by microRNA mimic-expressing adeno-associated virus (AAV-miRNA mimic) activated Rac1-PAK signaling, while Bcr-GAP-expressing AAV inactivated it. Furthermore, conditional knockdown of Bcr in the NAc of wild-type adult mice enhanced aversive learning, while Bcr-GAP expression in the NAc inhibited it. The findings indicate that Rac1 activation by RacGAP inhibition enhances aversive learning, implying the AD therapeutic potential of Rac1 signaling.

ITGB1BP1, a Novel Transcriptional Target of CD44-Downstream Signaling Promoting Cancer Cell Invasion

Breast cancer (BC) is the most common malignancy worldwide and has a poor prognosis, because it begins in the breast and disseminates to lymph nodes and distant organs. While invading, BC cells acquire aggressive characteristics from the tumor microenvironment through several mechanisms. Thus, understanding the mechanisms underlying the process of BC cell invasion can pave the way towards the development of targeted therapeutics focused on metastasis. We have previously reported that the activation of CD44 receptor with its major ligand hyaluronan (HA) promotes BC metastasis to the liver in vivo. Next, a gene expression profiling microarray analysis was conducted to identify and validate CD44-downstream transcriptional targets mediating its pro-metastatic function from RNA samples collected from Tet CD44-induced versus control MCF7-B5 cells. We have already validated a number of novel CD44-target genes and published their underlying signaling pathways in promoting BC cell invasion. From the same microarray analysis, Integrin subunit beta 1 binding protein 1 (ITGB1BP1) was also identified as a potential CD44-target gene that was upregulated (2-fold) upon HA activation of CD44. This report will review the lines of evidence collected from the literature to support our hypothesis, and further discuss the possible mechanisms linking HA activation of CD44 to its novel potential transcriptional target ITGB1BP1.

The anticancer phytochemical rocaglamide inhibits Rho GTPase activity and cancer cell migration

Chemotherapy is one of the pillars of anti-cancer therapy. Although chemotherapeutics cause regression of the primary tumor, many chemotherapeutics are often shown to induce or accelerate metastasis formation. Moreover, metastatic tumors are largely resistant against chemotherapy. As more than 90% of cancer patients die due to metastases and not due to primary tumor formation, novel drugs are needed to overcome these shortcomings. In this study, we identified the anticancer phytochemical Rocaglamide (Roc-A) to be an inhibitor of cancer cell migration, a crucial event in metastasis formation. We show that Roc-A inhibits cellular migration and invasion independently of its anti-proliferative and cytotoxic effects in different types of human cancer cells. Mechanistically, Roc-A treatment induces F-actin-based morphological changes in membrane protrusions. Further investigation of the molecular mechanisms revealed that Roc-A inhibits the activities of the small GTPases RhoA, Rac1 and Cdc42, the master regulators of cellular migration. Taken together, our results provide evidence that Roc-A may be a lead candidate for a new class of anticancer drugs that inhibit metastasis formation.

IQGAP1 Interaction with RHO Family Proteins Revisited: KINETIC AND EQUILIBRIUM EVIDENCE FOR MULTIPLE DISTINCT BINDING SITES

IQ motif-containing GTPase activating protein 1 (IQGAP1) plays a central role in the physical assembly of relevant signaling networks that are responsible for various cellular processes, including cell adhesion, polarity, and transmigration. The RHO family proteins CDC42 and RAC1 have been shown to mainly interact with the GAP-related domain (GRD) of IQGAP1. However, the role of its RASGAP C-terminal (RGCT) and C-terminal domains in the interactions with RHO proteins has remained obscure. Here, we demonstrate that IQGAP1 interactions with RHO proteins underlie a multiple-step binding mechanism: (i) a high affinity, GTP-dependent binding of RGCT to the switch regions of CDC42 or RAC1 and (ii) a very low affinity binding of GRD and a C terminus adjacent to the switch regions. These data were confirmed by phosphomimetic mutation of serine 1443 to glutamate within RGCT, which led to a significant reduction of IQGAP1 affinity for CDC42 and RAC1, clearly disclosing the critical role of RGCT for these interactions. Unlike CDC42, an extremely low affinity was determined for the RAC1-GRD interaction, suggesting that the molecular nature of IQGAP1 interaction with CDC42 partially differs from that of RAC1. Our study provides new insights into the interaction characteristics of IQGAP1 with RHO family proteins and highlights the complementary importance of kinetic and equilibrium analyses. We propose that the ability of IQGAP1 to interact with RHO proteins is based on a multiple-step binding process, which is a prerequisite for the dynamic functions of IQGAP1 as a scaffolding protein and a critical mechanism in temporal regulation and integration of IQGAP1-mediated cellular responses.

M3 muscarinic acetylcholine receptor dysfunction inhibits Rac1 activity and disrupts VE-cadherin/β-catenin and actin cytoskeleton interaction

The objective was to investigate whether M3 muscarinic acetylcholine receptor (mAChR) dysfunction disrupts the linkage between the vascular endothelial (VE)-cadherin in the adherens junctional complex and the actin-based cytoskeleton, increasing vascular permeability in atherosclerosis. Western blotting revealed that a selective M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), and M3 receptor siRNA decrease VE-cadherin and β-catenin in Triton X-100-insoluble fractions, indicating that M3 receptor inhibition weakens the linkage between the VE-cadherin/β-catenin complex and the actin cytoskeleton. Co-immunoprecipitation assays showed that M3 receptor inhibition reduces Rac1 activity and the association of IQ motif-containing GTPase-activating protein 1 (IQGAP1) with Ras-related C3 botulinum toxin substrate 1 (Rac1), while increasing the interaction between IQGAP1 and β-catenin. Using IQGAP1 siRNA, we found that IQGAP1 is required for stable interaction between VE-cadherin/β-catenin and the actin cytoskeleton in quiescent endothelial cells; IQGAP1 siRNA augments the M3 receptor inhibition-induced dissociation between them. Moreover, S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, attenuates this disassociation and Rac1 activity inhibition. The M3 receptor facilitates interaction of the VE-cadherin-based adherens junctional complex and the actin-based cytoskeleton by maintaining Rac1 activity, which regulates the interaction between IQGAP1/Rac1 and IQGAP1/β-catenin, and may contribute to endothelial barrier function under physiological conditions.

Structure and function of IQ-domain GTPase-activating protein 1 and its association with tumor progression (Review)

IQ-domain GTPase-activating proteins (IQGAPs) are evolutionary conserved multidomain proteins that are found in numerous organisms, from yeast to mammals. To date, three IQGAP proteins have been identified in humans, of which IQGAP1 is the best characterized. As a scaffold protein, IQGAP1 contains multiple protein-interacting domains, which modulate binding to target proteins. Recent mounting studies demonstrated a role for IQGAP1 in tumor progression, supported by the altered expression and subcellular distribution of IQGAP1 in tumors. The contribution of IQGAP1 to tumor progression appears to involve a complex interplay of cell functions by integrating diverse signal transduction pathways and coordinating activities, such as cell adhesion, migration, invasion, proliferation and angiogenesis.

IQGAP1: a regulator of intracellular spacetime relativity

Activating and inhibiting receptors of lymphocytes collect valuable information about their mikròs kósmos. This information is essential to initiate or to turn off complex signaling pathways. Irrespective of these advances, our knowledge on how these intracellular activation cascades are coordinated in a spatiotemporal manner is far from complete. Among multiple explanations, the scaffolding proteins have emerged as a critical piece of this evolutionary tangram. Among many, IQGAP1 is one of the essential scaffolding proteins that coordinate multiple signaling pathways. IQGAP1 possesses multiple protein interaction motifs to achieve its scaffolding functions. Using these domains, IQGAP1 has been shown to regulate a number of essential cellular events. This includes actin polymerization, tubulin multimerization, microtubule organizing center formation, calcium/calmodulin signaling, Pak/Raf/Mek1/2-mediated Erk1/2 activation, formation of maestrosome, E-cadherin, and CD44-mediated signaling and glycogen synthase kinase-3/adenomatous polyposis coli-mediated β-catenin activation. In this review, we summarize the recent developments and exciting new findings of cellular functions of IQGAP1.

The desmosome and pemphigus

Desmosomes are patch-like intercellular adhering junctions ("maculae adherentes"), which, in concert with the related adherens junctions, provide the mechanical strength to intercellular adhesion. Therefore, it is not surprising that desmosomes are abundant in tissues subjected to significant mechanical stress such as stratified epithelia and myocardium. Desmosomal adhesion is based on the Ca(2+)-dependent, homo- and heterophilic transinteraction of cadherin-type adhesion molecules. Desmosomal cadherins are anchored to the intermediate filament cytoskeleton by adaptor proteins of the armadillo and plakin families. Desmosomes are dynamic structures subjected to regulation and are therefore targets of signalling pathways, which control their molecular composition and adhesive properties. Moreover, evidence is emerging that desmosomal components themselves take part in outside-in signalling under physiologic and pathologic conditions. Disturbed desmosomal adhesion contributes to the pathogenesis of a number of diseases such as pemphigus, which is caused by autoantibodies against desmosomal cadherins. Beside pemphigus, desmosome-associated diseases are caused by other mechanisms such as genetic defects or bacterial toxins. Because most of these diseases affect the skin, desmosomes are interesting not only for cell biologists who are inspired by their complex structure and molecular composition, but also for clinical physicians who are confronted with patients suffering from severe blistering skin diseases such as pemphigus. To develop disease-specific therapeutic approaches, more insights into the molecular composition and regulation of desmosomes are required.

M2 and M3 muscarinic receptor activation of urinary bladder contractile signal transduction. I. Normal rat bladder

The muscarinic receptor subtype-activated signal transduction mechanisms mediating rat urinary bladder contraction are incompletely understood. M(3) mediates normal rat bladder contractions; however, the M(2) receptor subtype has a more dominant role in contractions of the hypertrophied bladder. Normal bladder muscle strips were exposed to inhibitors of enzymes thought to be involved in signal transduction in vitro followed by a single cumulative concentration-response curve to the muscarinic receptor agonist carbachol. The outcome measures were the maximal contraction, the potency of carbachol, and the affinity of the M(3) -selective antimuscarinic agent darifenacin for inhibition of contraction. Inhibition of phosphoinositide-specific phospholipase C (PI-PLC) with 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH(3)) reduces carbachol potency and reduces darifenacin affinity, whereas inhibition of phosphatidyl choline-specific phospholipase C (PC-PLC) with O-tricyclo[5.2.1.02,6]dec-9-yl dithiocarbonate potassium salt (D609) attenuates the carbachol maximal contraction. Inhibition of rho kinase with (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclohexanecarboxamide dihydrochloride (Y-27632) reduces carbachol potency and increases darifenacin affinity. Inhibition of rho kinase, protein kinase A (PKA), and protein kinase G (PKG) with 1-(5-isoquinolinesulfonyl)-homopiperazine.HCl (HA-1077) reduces the carbachol maximal contraction, carbachol potency, and darifenacin affinity. Inhibition of protein kinase C (PKC) with chelerythrine increases darifenacin affinity, whereas inhibition of rho kinase, PKA, PKG, and PKC with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine.2HCl (H7) reduces the carbachol maximum and carbachol potency while increasing darifenacin affinity. Inhibition of rho kinase, PKA, and PKG with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl (H89) reduces carbachol maximum and carbachol potency. Both the M(2) and the M(3) receptor subtype are involved in normal rat bladder contractions. The M(3)subtype seems to mediate contraction by activation of PI-PLC, PC-PLC, and PKA, whereas the M(2) signal transduction cascade may include activation of rho kinase, PKC, and an additional contractile signal transduction mechanism independent of rho kinase or PKC.

IQGAP1 is a scaffold for mitogen-activated protein kinase signaling

IQGAP1 modulates many cellular functions such as cell-cell adhesion, transcription, cytoskeletal architecture, and selected signaling pathways. We previously documented that IQGAP1 binds extracellular signal-regulated kinase (ERK) 2 and regulates growth factor-stimulated ERK activity. Here we show that MEK, the molecule immediately upstream of ERK in the Ras/mitogen-activated protein (MAP) kinase signaling cascade, also interacts directly with IQGAP1. Both MEK1 and MEK2 bound IQGAP1 in vitro and coimmunoprecipitated with IQGAP1. The addition of ERK2 enhanced by fourfold the in vitro interaction of MEK2 with IQGAP1 without altering binding of MEK1. Similarly, ERK1 promoted MEK binding to IQGAP1, but either MEK protein altered the association between IQGAP1 and ERK. Epidermal growth factor (EGF) differentially regulated binding, enhancing MEK1 interaction while reducing MEK2 binding to IQGAP1. In addition, both knockdown and overexpression of IQGAP1 reduced EGF-stimulated activation of MEK and ERK. Analyses with selective IQGAP1 mutant constructs indicated that MEK binding is crucial for IQGAP1 to modulate EGF activation of ERK. Our data strongly suggest that IQGAP1 functions as a molecular scaffold in the Ras/MAP kinase pathway.

IQGAPs are differentially expressed and regulated in polarized gastric epithelial cells

IQGAPs, GTPase-activating proteins with an IQ motif, are thought to regulate many actin cytoskeleton-based activities through interactions with Cdc42 and Rac. Recently, Cdc42 was implicated in regulation of gastric parietal cell HCl secretion, and IQGAP2 was immunolocalized with Cdc42 to F-actin-rich intracellular canalicular membranes of isolated gastric parietal cells in primary culture. Here we sought to define distribution and localization of IQGAP1 and IQGAP2 in major oxyntic (acid-secreting) gastric mucosal cell types and to determine whether secretory agonists modulate these proteins. Differential staining protocols were used to identify different cell populations (parietal, chief, surface/pit, and mucous neck cells) in semi-intact glands isolated from rabbit gastric mucosae and to characterize these same cells after dispersion and fractionation on isopycnic density gradients with simultaneous staining for F-actin, H+-K+-ATPase, and GSII lectin-binding sites. There was a pronounced increase in intracellular F-actin staining in dispersed chief cells, apparently from internalization of F-actin-rich apical membranes that normally abut the gland lumen. Therefore, other membrane-associated proteins might also be redistributed by disruption of cell-cell contacts. Western blot analyses were used to quantitate relative concentrations of IQGAPs in defined mucosal cell fractions, and gastric glands were used for in situ localizations. We detected uniform levels of IQGAP2 expression in oxyntic mucosal cells with predominant targeting to regions of cell-cell contact and nuclei of all cell types. IQGAP2 was not detected in parietal cell intracellular canaliculi. IQGAP1 expression was variable and targeted predominantly to the cortex of chief and mucous neck cells. Parietal cells expressed little or no IQGAP1 vs. other mucosal cell types. Phosphoprotein affinity chromatography, isoelectric focusing, and phosphorylation site analyses indicated that both IQGAP1 and IQGAP2 are phosphoproteins potentially regulated by [Ca2+]i/PKC and cAMP signaling pathways, respectively. Stimulation of glands with carbachol, which elevates [Ca2+]i and activates PKC, induced apparent translocation of IQGAP1, but not IQGAP2, to apical poles of chief (zymogen) and mucous neck cells. This response was mimicked by PMA but not by ionomycin or by elevation of [cAMP]i with forskolin. Our observations support a novel, PKC-dependent role for IQGAP1 in regulated exocytosis and suggest that IQGAP2 may play a more general role in regulating cell-cell interactions and possibly migration within the gastric mucosa.

P-REX2, a novel PI-3-kinase sensitive Rac exchange factor

We have identified an activator of Rac, P-REX2, that is structurally related to the exchange factor PtdIns(3,4,5)-dependent Rac exchanger (P-REX1), but exhibits distinct tissue-specific expression. P-REX2 is spliced into two RNA species, approximately 3.5 and approximately 10 kb in size. The cDNA corresponding to the smaller transcript encodes a protein that exhibits strong similarity with P-REX1 within its N-terminal domains, but differs in the C-terminal region. P-REX2 promoted increased levels of GTP-bound Rac that could be further stimulated by enhancing PI-3K activity. Thus, P-REX2 may serve as a novel link between Rac activation and the PI-3 kinase pathway.

The Rac1 C-terminal polybasic region regulates the nuclear localization and protein degradation of Rac1

We observed evolutionary conservation of canonical nuclear localization signal sequences (K(K/R)X(K/R)) in the C-terminal polybasic regions (PBRs) of some Rac and Rho isoforms. Canonical D-box sequences (RXXL), which target proteins for proteasome-mediated degradation, are also evolutionarily conserved near the PBRs of these small GTPases. We show that the Rac1 PBR (PVKKRKRK) promotes Rac1 nuclear accumulation, whereas the RhoA PBR (RRGKKKSG) keeps RhoA in the cytoplasm. A mutant Rac1 protein named Rac1 (pbrRhoA), in which the RhoA PBR replaces the Rac1 PBR, has greater cytoplasmic localization, enhanced resistance to proteasome-mediated degradation, and higher protein levels than Rac1. Mutating the D-box by substituting alanines at amino acids 174 and 177 significantly increases the protein levels of Rac1 but not Rac1(pbrRhoA). These results suggest that Rac1 (pbrRhoA) is more resistant than Rac1 to proteasome-mediated degradative pathways involving the D-box. The cytoplasmic localization of Rac1(pbrRhoA) provides the most obvious reason for its resistance to proteasome-mediated degradation, because we show that Rac1(pbrRhoA) does not greatly differ from Rac1 in its ability to stimulate membrane ruffling or to interact with SmgGDS and IQGAP1-calmodulin complexes. These findings support the model that nuclear localization signal sequences in the PBR direct Rac1 to the nucleus, where Rac1 participates in signaling pathways that ultimately target it for degradation.

Elevated Rac1 activity changes the M3 muscarinic acetylcholine receptor-mediated inhibition of proliferation to induction of cell death

Although muscarinic acetylcholine receptors (mAChRs) regulate proliferation in many cell types, the signaling pathways involved are unclear. The participation of the small GTPases Rac1 and RhoA in M(3) mAChR-mediated inhibition of proliferation was investigated by activating M(3) mAChRs stably transfected in Chinese hamster ovary cells stably coexpressing hemagglutinin (HA)-tagged wild-type or mutant Rac1 or RhoA proteins. Activation of M(3) mAChRs activates both Rac1 and RhoA and inhibits cell proliferation in all cell lines tested. mAChR-mediated inhibition of proliferation is diminished in cells expressing dominant-negative HA-Rac1(Asn17) (m3DNRac) but is enhanced in cells expressing HA-Rac1 (m3WTRac) or constitutively active HA-Rac(Val12) (m3CARac). The activation of mAChRs in m3WTRac and m3CARac cells also induces apoptosis. Expression of wild-type or mutant RhoA proteins does not alter mAChR-mediated inhibition of proliferation. mAChR-induced inhibition of proliferation is abrogated in all cell lines when Galpha(q/11) signaling is terminated by transient expression of the COOH-terminal fragment of phospholipase C (PLC-beta1ct), the NH(2)-terminal fragment of G protein-coupled receptor kinase, or the regulator of G protein signaling 2. Pretreatment of all cells expressing wild-type or mutant Rac1 proteins with edelfosine, a phosphatidylinositol-specific PLC inhibitor, or Go 6976, which inhibits conventional protein kinase C (PKC) isoforms, diminishes the M(3) mAChR's ability to inhibit proliferation. Our results identify Galpha(q/11), PLC, and PKC as participants in the M(3) mAChR-mediated inhibition of cell proliferation. These findings indicate that in the context of high Rac1 activity, but not RhoA activity, M(3) mAChR-mediated activation of these participants triggers cell death.

Pemphigus Vulgaris IgG and Methylprednisolone Exhibit Reciprocal Effects on Keratinocytes

Pemphigus vulgaris (PV) is a life-threatening autoimmune disease of skin adhesion associated with IgG autoantibodies against keratinocytes (KC). Treatment of PV with systemic corticosteroids is life-saving, but the mechanism of the therapeutic action has not been fully understood. We have developed an animal model that demonstrates that methylprednisolone (MP) can block PV IgG-induced acantholysis, decreasing the extent of keratinocyte detachment in the epidermis of 3-5-day-old nude mice from 77.5 +/- 0.6 to 24.1 +/- 1.5% (p < 0.05). We hypothesized that in addition to immunosuppression, MP may exhibit direct anti-acantholytic effects in epidermis, and we compared the effects of PV IgG and MP on KC. The use of DNA microarray showed that PV IgG down-regulated and MP up-regulated expression of the genes encoding keratinocyte adhesion molecules, antigen-processing proteins, regulators of cell cycle and apoptosis, differentiation markers, Na+,K+-ATPase, protein kinases and phosphatases, and serine proteases and their inhibitors. Overall, PV IgG decreased transcription of 198 genes and increased transcription of 31 genes. MP decreased transcription of 14 genes and increased transcription of 818 genes. Specific effects of PV IgG and MP on keratinocyte adhesion molecules were further investigated by Western blot and immunofluorescence assays. By immunoblotting, MP increased the protein levels of E-cadherin and desmogleins 1 and 3 by 300, 180, and 40%, respectively. Specific staining of KC for E-cadherin and desmogleins 1 and 3 increased by 235, 228, and 148%, respectively. In addition, PV IgG increased the level of phosphorylation of E-cadherin by 42%, beta-catenin by 37%, gamma-catenin by 136%, and desmoglein 3 by 300%, whereas pretreatment with 0.25 mm MP abolished phosphorylation of these adhesion molecules. These results suggested that therapeutic effects of MP in PV include both the up-regulated synthesis and post-translational modification of the keratinocyte adhesion molecules.

Conserved microtubule-actin interactions in cell movement and morphogenesis

Interactions between microtubules and actin are a basic phenomenon that underlies many fundamental processes in which dynamic cellular asymmetries need to be established and maintained. These are processes as diverse as cell motility, neuronal pathfinding, cellular wound healing, cell division and cortical flow. Microtubules and actin exhibit two mechanistic classes of interactions--regulatory and structural. These interactions comprise at least three conserved 'mechanochemical activity modules' that perform similar roles in these diverse cell functions.

IQGAP1 as signal integrator: Ca2+, calmodulin, Cdc42 and the cytoskeleton

A family of proteins known as IQGAPs have been identified in yeast, amebas and mammals. IQGAPs are multidomain molecules that contain several protein-interacting motifs which mediate binding to target proteins. Mammalian IQGAP1 is a component of signaling networks that are integral to maintaining cytoskeletal architecture and cell-cell adhesion. Published data suggest that IQGAP1 is a scaffolding protein that modulates cross-talk among diverse pathways in complex regulatory circuits. These pathways include modulating the actin cytoskeleton, mediating signaling by Rho family GTPases and calmodulin, regulating E-cadherin and beta-catenin function and organizing microtubules.

Novel mechanism of the co-regulation of nuclear transport of SmgGDS and Rac1

The armadillo protein SmgGDS promotes guanine nucleotide exchange by small GTPases containing a C-terminal polybasic region (PBR), such as Rac1 and RhoA. Because the PBR resembles a nuclear localization signal (NLS) sequence, we investigated the nuclear transport of SmgGDS with Rac1 or RhoA. We show that the Rac1 PBR has significant NLS activity when it is fused to green fluorescent protein (GFP) or in the context of full-length Rac1. In contrast, the RhoA PBR has very poor NLS activity when it is fused to GFP or in the context of full-length RhoA. The nuclear accumulation of both Rac1 and SmgGDS is enhanced by Rac1 activation and diminished by mutation of the Rac1 PBR. Conversely, SmgGDS nuclear accumulation is diminished by interactions with RhoA. An SmgGDS nuclear export signal sequence that we identified promotes SmgGDS nuclear export. These results suggest that SmgGDS. Rac1 complexes accumulate in the nucleus because the Rac1 PBR has NLS activity and because Rac1 supplies the appropriate GTP-dependent signal. In contrast, SmgGDS.RhoA complexes accumulate in the cytoplasm because the RhoA PBR does not have NLS activity. This model may be applicable to other armadillo proteins in addition to SmgGDS, because we demonstrate that activated Rac1 and RhoA also provide stimulatory and inhibitory signals, respectively, for the nuclear accumulation of p120 catenin. These results indicate that small GTPases with a PBR can regulate the nuclear transport of armadillo proteins.

Muscarinic signaling in carcinoma cells

We previously reported that activation of M(3) muscarinic acetylcholine receptors (mAChR) generates anti-proliferative signals and stimulates cadherin-mediated adhesion in the SCC-9 small cell lung carcinoma (SCLC) cell line. The current study was undertaken to determine the frequency of functional mAChR expression among different SCLC cell lines, and to test the ability of mAChR to generate anti-proliferative signals in different SCLC cell lines. The potential role of Rac1 in SCLC cell-cell adhesion was also investigated. Exposure to the mAChR agonist carbachol induces robust Ca(2+) mobilization (indicated by intracellular fluorescence of the Ca(2+)-binding dye Indo-1) in three SCLC cell lines (SCC-9, SCC-15, and NCI-H146), modest Ca(2+) mobilization in one SCLC cell line (NCI-H209), and no detectable Ca(2+) mobilization in two SCLC cell lines (SCC-18 and NCI-H82). The M(3) mAChR-selective antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide inhibits Ca(2+) mobilization in all SCLC cell lines responding to carbachol. Incubation with carbachol for four hours significantly inhibits [3H]thymidine uptake in three of the four SCLC cell lines expressing functional mAChR (SCC-9, SCC-15, and NCI-H146 cells), but does not significantly alter [3H]thymidine uptake in the other SCLC cell lines examined. These results indicate that SCLC cell lines often express functional mAChR which elicit anti-proliferative signals when activated. To investigate the role of Rac1 in SCLC adhesion, SCC-9 cells were transiently transfected with cDNA constructs coding for Rac1, constitutively active Rac1(Val-12), or dominant negative Rac1(Asn-17) tagged to green fluorescent protein (GFP). SCC-9 cells expressing GFP-tagged constitutively active Rac1(Val-12) exhibit increased cell-cell adhesion in comparison to cells expressing GFP-Rac1 or GFP-Rac1(Asn-17). Constitutively active GFP-Rac1(Val-12), but not GFP-Rac1 or GFP-Rac1(Asn-17), accumulates at cell-cell junctions in SCC-9 cells. These results indicate that activated Rac1 increases SCLC cell-cell adhesion, consistent with the possibility that Rac1 activation contributes to increased SCLC cell-cell adhesion induced by mAChR stimulation. These findings indicate that activation of mAChR may play a significant role in regulating the proliferation and adhesion of SCLC cells. The demonstration by other investigators that acetylcholine is expressed by a variety of cells in the airways supports the possibility that acetylcholine may activate mAChR expressed by SCLC cells in primary tumors.